Embodiments of the present invention relate to the field of semiconductor devices. Specifically, embodiments of the present invention relate to a heterojunction semiconductor device having an intermediate layer for improving a junction.
Heterojunction bipolar transistors (HBTs) have become state of the art, particularly in npn form, for applications in which high switching speeds and high frequency operation are desired. The emitter in an HBT has a bandgap wider than the bandgap of the base, thus creating an energy barrier in the valence band at the emitter-base junction that inhibits the unwanted flow of holes from the base region to the emitter region. This arrangement increases the emitter injection efficiency, current gain, and operating frequency of the HBT.
First generation commercial HBTs were based on a gallium-arsenide (GaAs) substrate and semiconductor materials lattice matched to GaAs. Next generation HBTs are likely to be based on an indium-phosphide (InP) substrate and semiconductor materials lattice matched to InP. Typically, the base of such an HBT is fabricated from either the indium-gallium-arsenide (InGaAs) material system or the gallium-arsenide-antimonide (GaAsSb) material system, with the collector and the emitter fabricated from, for example, InP or InGaAs.
Heterojunction bipolar transistors (HBTs) with a GaAsSb base region and an InP emitter region have been grown epitaxially using metalorganic vapor phase epitaxy (MOVPE) for many years. It has been known that the performance of HBTs is greatly affected by the quality of the junction between the GaAsSb base region and the InP emitter region. Achieving the high quality junction between the GaAsSb base region and the InP emitter region is a challenging task since the elemental species of both group III and V atoms changes at the junction between the two regions.
A conventional way of improving the junction quality is optimizing the gas-switching sequence and utilizing growth pauses between gas switching. However, this approach involves time-consuming investigation of numerous gas-switching sequences resulting from all possible combinations of five different constituent atoms at the junction and variable growth-pause times that can be placed at the growth stops. Furthermore, the gas-switching sequence optimized in one MOVPE reactor cannot be easily transferred to another MOVPE reactor unless the reactors have a nearly identical reactor hardware design. This is because a reactor""s hardware design has a direct influence on the complex gas-flow dynamics within an MOVPE reactor, which in turn dictates the gas-switching sequence.
As mentioned above, the performance of HBTs is greatly affected by the quality of the junction between the GaAsSb base region and the InP emitter region. Defects present in the junction negatively impact performance by causing recombination of charge carriers. Thus, unless very complex and time-consuming epitaxial growth steps are taken, the performance of the HBT will suffer.
Thus, one problem with conventional methods for forming a heterojunction in a semiconductor device is that it is difficult to form a junction with good properties without resorting to complex epitaxial growth steps. Moreover, another problem with conventional methods is that they are highly dependent on the hardware design of the MOVPE reactor. A still further problem is conventional HBTs suffer from excessive recombination of charge carriers.
The present invention pertains to a heterojunction for a semiconductor device. An embodiment provides a semiconductor junction comprising a first region formed from a first semiconductor material and having a first conductivity type, a second region formed from a second semiconductor material and having a second conductivity type, and an intermediate layer between the first region and the second region. The band line-up of the first region, the intermediate layer, and the second region has no bound states in its conduction band and no bound states in its valence band. In some embodiments, the intermediate layer has a thickness small enough to allow electrons to tunnel from the first region to the second region with negligible attenuation.
Another embodiment provides for a heterojunction bipolar transistor comprising a collector region and a base region forming a junction with the collector region. The transistor also comprises an emitter region and an intermediate layer between the base region and the emitter region. The intermediate layer has a higher conduction band energy level than the conduction band energy level of the emitter region. Furthermore, the intermediate layer is thin enough for electrons to tunnel from the emitter region to the base region with negligible attenuation.
Another embodiment provides for a method for forming a heterojunction. The method includes forming a first region comprising a first semiconductor material having a first conductivity type. The method further includes forming an intermediate layer proximate the first region. The method also includes forming a second region proximate the intermediate layer and comprising a second semiconductor material having a second conductivity type, wherein there are no bound states formed in either the conduction or the valence band of the heterojunction.